The stack trace shows libgallium-26.0.3 which is part of ubuntu not the AppImage that is crashing.
Your system libgallium is being pulled in by libQtWebEngineCore from the AppImage.
Code:
Stack trace of thread 10786:
#0 0x000074933e49246c n/a (libgallium-26.0.3-1ubuntu1.so + 0x129246c)
#1 0x000074933d40639d n/a (libgallium-26.0.3-1ubuntu1.so + 0x20639d)
#2 0x000074933d3f1ea8 n/a (libgallium-26.0.3-1ubuntu1.so + 0x1f1ea8)
#3 0x000074933d6b819f n/a (libgallium-26.0.3-1ubuntu1.so + 0x4b819f)
#4 0x000074933d6b910f n/a (libgallium-26.0.3-1ubuntu1.so + 0x4b910f)
#5 0x0000749355caa405 n/a (/tmp/.mount_Sigil-MGADof/usr/lib/libQt6WebEngineCore.so.6 + 0xcaa405)
#6 0x0000749355c6943e n/a (/tmp/.mount_Sigil-MGADof/usr/lib/libQt6WebEngineCore.so.6 + 0xc6943e)
#7 0x00007493526d4948 n/a (/tmp/.mount_Sigil-MGADof/usr/lib/libQt6Quick.so.6 + 0x2d4948)
ELF object binary architecture: AMD x86-64
So I would try to downgrade that ubuntu libgallium package and see if the problem still exists. My guess is it is the problem.