JOSM asks to save unmodified GPX files when opening them through a saved session

[xeen]

steps to reproduce:

1. Start JOSM
2. open any GPX file
3. enable expert mode
4. save session as
5. close JOSM. Observe: no dialog to save GPX
6. Start JOSM again
7. open previously saved session
8. close JOSM. Observe: it asks to save/upload the supposedly modified GPX file

Note that the STR above are just one way to trigger this bug. As far as I've tested, it is triggered for any GPX file opened through a saved session, and how you arrive there is not important (i.e. the .jos file doesn't have a wrong attribute on the <layer> tag for the GPX file).

Status report below is with a fresh ~/.josm directory.

Revision:17702
Is-Local-Build:false
Build-Date:1969-12-10 07:20:05
Debian-Release:0.0.svn17702+dfsg-1~exp1
Build-Name:Debian

Identification: JOSM/1.5 (17702 Debian en) Linux Debian GNU/Linux bullseye/sid
Memory Usage: 350 MB / 4096 MB (255 MB allocated, but free)
Java version: 13.0.5.1+1-Debian-1, Debian, OpenJDK 64-Bit Server VM
Look and Feel: javax.swing.plaf.metal.MetalLookAndFeel
Screen: :0.0 3840×2160 (scaling 2.00×2.00)
Maximum Screen Size: 3840×2160
Best cursor sizes: 16×16→16×16, 32×32→32×32
Environment variable LANG: en_US.UTF-8
System property file.encoding: UTF-8
System property sun.jnu.encoding: UTF-8
Java ATK Wrapper package: libatk-wrapper-java:all-0.38.0-2
libcommons-compress-java: libcommons-compress-java:all-1.20-1
libcommons-logging-java: libcommons-logging-java:all-1.2-2
fonts-noto: fonts-noto:all-20201225-1
liboauth-signpost-java: liboauth-signpost-java:all-1.2.1.2-3
VM arguments: [-Djosm.restart=true, -Djava.net.useSystemProxies=true]

[skyper]

Note: Debian is in freeze, atm, which means even backport packages are not upgraded.

Does this also happen with our own josm-latest (ubuntu) deb-package, which can be installed along side the official tested josm package from Debian, see Download.