Qgis: Layout -> Legend -> when using diagrams (Pie Chart), weird rendering

[hshatti]

Describe the bug

The legend doesn't render a pie diagram correctly instead it shows a weird placement in the front-end and the hides the correct one in the back
How to Reproduce

1. Add any shapefile a number feature and link it to a [Pie Chart] Diagram in the layer properties

2. from the Diagrams page select legend and configure a collapsed or Separated legend [example]:
   

3. Go to 'Project'

4. Click on 'New Print Layout'
5. Add a Map containing a Pie Chart diagram, then add a Legend
6. See error

please check also the separated render and other charts if they render correctly in the layout.

QGIS and OS versions
3.12 / Windows 10 (64bit)

QGIS version
3.12.0-București
QGIS code revision
cd141490ec
Compiled against Qt
5.11.2
Running against Qt
5.11.2
Compiled against GDAL/OGR
3.0.4
Running against GDAL/OGR
3.0.4
Compiled against GEOS
3.8.0-CAPI-1.13.1
Running against GEOS
3.8.0-CAPI-1.13.1
Compiled against SQLite
3.29.0
Running against SQLite
3.29.0
PostgreSQL Client Version
11.5
SpatiaLite Version
4.3.0
QWT Version
6.1.3
QScintilla2 Version
2.10.8
Compiled against PROJ
6.3.1
Running against PROJ
Rel. 6.3.1, February 10th, 2020
OS Version
Windows 10 (10.0)
Active python plugins
d3datavis;
dataexplorer;
openlayers_plugin;
OSMDownloader;
OSMtools;
qconsolidate3;
shapetools;
VectorBender;
db_manager;
MetaSearch;
processing

Additional context

[arongergely]

I have the same same issue on Ubuntu Bionic, QGIS v3.12.1!

But I noticed something else too that could help to troubleshoot.

First, this is my buggy legend:

If I disable the legend background then tick on Resize to fit contents in the legend main properties, I seem to get a duplicate of the items rendered over the original, all inside the same legend box... except the duplicate pie chart item is rendered correctly:

[roya0045]

I tried to test this, and I think I missed a step. I can't validate this issue as my legend just becomes a damn giraffe for no reason in 3.12 on windows.

legend_giraffe.zip

[Benidrissa]

After trying @arongergely workaround
"If I disable the legend background then tick on Resize to fit contents in the legend main properties, I seem to get a duplicate of the items rendered over the original, all inside the same legend box... except the duplicate pie chart item is rendered correctly:"

There is no duplicated legends when you export to PDF, image, ...
I have the same problem of superimposing proportional circles in a legend with version 3.12.1.
I see that it is correct when exported.