Some WP.com themes add Tags to Pages and Portfolio Projects, usually for featured content. When a Tag is added to a Portfolio Project, it is converted to a string of numbers. I was able to reproduce on WP.com simple sites, but not AT sites.
I expected the Portfolio Project to have the Tag I typed in.
The Tag I typed in was converted to a string of numbers.
Chrome Version 72.0.3626.119 (Official Build) (64-bit)
macOS Mojave version 10.14.3

I ran into this as well on 11032083-hc (1861597-zen), but for pages. I tested extensively and it happens on:
It does not happen for pages created on:
WordPress.com Business plan site used for all tests.
Google Chrome Version 72.0.3626.109 (Official Build) (64-bit) with macOS 10.14.2 - Mac OS Mojave, if that is important.
10938062-hc / cc @nagpai's report.
Just had a case myself in 11048610-hc
Another one reported here: 11147919-HC
In 1858292-zen: feedback that this bug is impacting SEO
I've got another one here: 7072281-HC
I did find another workaround:
That worked as a temporary workaround on this particular site.
Another reported here:
11191758-hc
Another one here: 8088196-hc
Another here: 11156634-hc
Initial report was in #1857763-zen
1880190-zen
User provided some step-by-step screenshots to reproduce.
Re-shared the workaround by @sstabrizi here #1858292-zen and closed until it gets fixed.
Another reported here: 11249841-hc
I tested this on my own site and also found that once it changes it to a number, it won't let me delete it.
I can recreate it by creating a new page as shown here: https://cld.wthms.co/I6VGFs
I have the Eris theme on a Premium site.
Another case here 1883377-zen. Shared the temporary workaround from above.
Another report in 1884795-zen.
Another report in 1885144-zen
Verified:
I think something's up with the POST API calls to https://public-api.wordpress.com/wp/v2/sites/:siteId/tags.
Tags like these are getting created, then faithfully brought back to the interface (as opposed to it being an issue all in the UI).

Duplicate tags are being created with the name of the tag being the ID of the already-created tag.
Enter. The tag is added to the db via POST https://public-api.wordpress.com/wp/v2/sites/:siteId/tags?_envelope=1&_locale=user.PUT https://public-api.wordpress.com/wp/v2/sites/:siteId/jetpack-portfolio/:postId?_envelope=1&_locale=user.
Hm, maybe related complications and background: p7jreA-20e-p2.
Step 2 above (with the form data) looks like expected behaviour, since this is also how the working tags-on-posts performs. So the likely culprit is that PUT updates calling https://public-api.wordpress.com/wp/v2/sites/:siteId/jetpack-portfolio/postId (or maybe any CPT plus pages) are creating the tag[] value as a new name, rather than using them properly as tag IDs.
another issue on 9992877-HC
The problem could be the ID coming through as a string, at least it is when it gets to wp_set_post_terms.
Another issue: #11384762-hc
Another case of the same in 11408120-hc / 1894957-zen (for a page).
Another case of pages losing their tags and switching to numbers in 9921188-hc
Another report in 10172122-hc
+1 11455896-hc
Portfolio projects with Mellow theme
Another report.
Homepage tags - Heart And Style theme
11476631-hc
Another report:
1903240-zen
Another report in 1903510-zen
Another report: 1903794-zen
Confirmed it only happens on the Gutenberg editor. Replicated it with the "Caldwell" theme. I could only reproduce it with pages.
another: 9566559-hc
11597559-hc
Theme: Arcane
Happens on Pages.
When we enter even the same converted tag number from Calypso/iFramed Gutenberg, the number was turned to be another number again.
So it won't work at all there.
I edited the Page from WP Admin and it works just fine there.
Fix by @mdawaffe is underway on D26092-code.
Adding another report for posterity: https://fr.forums.wordpress.com/topic/probleme-etiquettes/
I believe this is now fixed. New updates to pages, portfolios, etc. should work correctly.
Most helpful comment
Fix by @mdawaffe is underway on D26092-code.