When uploading a file in chunked mode it now complains that the chunk is too small, but only with the second chunk. I was able to upload the first chunk with only 512 bytes.
I also checked to upload the first chunk with 1024 bytes and the second with less than 1024 to verify that it's not actually allowing one smaller chunk (instead of just allowing the last one).
I was using the API via Pywikibot and tried to upload a file in 512 byte chunks which did work until recently. Interestingly all tests in Gerrit 234851 worked except the chunked (as of PS5) and others also upload in 512 byte chunks, but just the first chunk.