Found a failing `debug_assert` in MTUD

[p-avital]

Hi,

Our CI just caught a panic caused by this assertion.

I haven't started digging yet as to the severity, and it's the only case we've observed in a few months of development, so it doesn't look like the conditions that trigger it are likely; but I'm raising the issue in case it raises flags for someone :)

I'll try to investigate if I get a gap to do so, but if anyone familiar with the code could leave me some hints, I'd be grateful :)