Wow, this is an incredibly specific bug ! 😲
Nice catch !

For the record I did not write that code, and I do not fully understand this method and the left shift operator.

If you did understand this:

• could you explain what you did understand ?
• do you have another reproductible cas as I could not reproduce the issue with other cases than 287 and 33

Hello @jdeniau !
I guess you were right to ask, my PR is far from being perfect. I have just noticed some cases were not handled. I will fix that shortly. Also I might have found another issue in the removeBefore method.
To answer your question, I wanted to understand deeply the algorithm and the data structures used under the cover by ImmutableJS. Hence I have started to reimplement an immutable list from scratch (https://github.com/alexvictoor/ts-immutable).
A few weeks ago I had the feeling that my implementation had the main features and I started to compare the behaviours of the main methods with the original ImmutableJS List using property based testing and wonderful fast-check library (thanks @dubzzz). I found a lot of bugs in my implementation using fast-check and then I have found this bug :)
Might be difficult to explain the code incriminated. If you like, since we are both french, we can schedule a google meet. FYI I am willing to explain how ImmutableJS works under the cover at Paris Typescript meetup in a few months

@alexvictoor great, I sent you an invite.
For the record I have the secret dream of converting immutable to TS 🤞

@alexvictoor For the record, I did add the same test as v but testing the 10 000 first integers, and here is the list of the number that was failing, and that do pass now with your code:

33, 42, 95, 127, 159, 191, 223, 255, 287, 319, 351, 383, 415, 447, 479, 511, 543, 575, 607, 639, 671, 703, 735, 767, 799, 831, 863, 895, 927, 959, 991, 1023

released in 5.0.3