When changing the dropdown in the property pane, this doesn't trigger the Apply button to get enabled like it is for other fields.
Here is an example of what is going on:

VSO 239943
Yeah, this is a known issue. This will be fixed in the next release. Thanks for pointing it out though.
Dropdown disable/enable feature is now fixed and will be in the next drop.
@srideshpande What is a "drop" ?
@srideshpande it's indeed fixed in the latest version.
@srideshpande I am still getting this issue, running on the latest version.
Hello @curtis-hennessy-rp - Thanks for the feedback. I just verified with in-house web parts and I am not seeing the issue. Could you please show/share how you are building the property pane configuration object which you pass it to the framework? and also did you see this issue in the latest release? or it was never fixed for you?
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Yeah, this is a known issue. This will be fixed in the next release. Thanks for pointing it out though.