update multipolygon action removes tags from ways without adding them to relation

[GerdP]

What steps will reproduce the problem?

1. Create new relation and add tag type=multipolygon
2. Select the new relation and a building way
3. Use Ctrl+Shift+B to update the multipolygon

What is the expected result?

tags from way are removed from the way and added to the relation

What happens instead?

tags from way are removed from the way and not added to the relation

Please provide any additional information below. Attach a screenshot if possible.

Build-Date:2020-12-12 14:42:33
Revision:17402
Is-Local-Build:true

Identification: JOSM/1.5 (17402 SVN en) Windows 10 64-Bit
OS Build number: Windows 10 Home 2004 (19041)
Memory Usage: 767 MB / 1820 MB (371 MB allocated, but free)
Java version: 1.8.0_272-b10, AdoptOpenJDK, OpenJDK 64-Bit Server VM
Look and Feel: com.sun.java.swing.plaf.windows.WindowsLookAndFeel
Screen: \Display0 1920×1080 (scaling 1.00×1.00)
Maximum Screen Size: 1920×1080
Best cursor sizes: 16×16→32×32, 32×32→32×32
VM arguments: [-agentlib:jdwp=transport=dt_socket,suspend=y,address=localhost:57586, -ea, -javaagent:D:\eclipse-java-2020-09\eclipse\configuration\org.eclipse.osgi\215\0\.cp\lib\javaagent-shaded.jar, -Dfile.encoding=UTF-8]
Dataset consistency test: No problems found

Plugins:
+ ColumbusCSV (35640)
+ buildings_tools (35669)
+ o5m (35640)
+ pbf (35650)
+ poly (35640)
+ reltoolbox (35640)
+ reverter (35640)
+ undelete (35640)
+ utilsplugin2 (35671)

Validator rules:
+ c:\josm\core\resources\data\validator\geometry.mapcss