Pattern coverage anti-test for persistent data fails on Trixie

In the Tails erases memory and shuts down on USB boot medium removal: persistent data scenario we sometimes (maybe 20% of runs) have this failure on Tails/Trixie:

    And I fill a 128 MiB file with a known pattern on the persistent filesystem (passed)                                   # features/step_definitions/erase_memory.rb:235
    And patterns cover at least 100 MiB in the guest's memory (failed)                                                     # features/step_definitions/erase_memory.rb:182
      Pattern coverage: 66.504% (66 MiB out of 100 MiB reference memory)
      66.504% of the expected size (100 MiB) has the pattern, but more than 100.000% was expected.

I've seen it with a wide range of coverage, from 65.359% to 98.984%.