Thank you for your notice, I found the error.
I used the ZX Spectrum part of the ep128 emulator. It works fine, it can load .tap and .tzx files, but... it I save, the .tap file become corrupt. The ep128 emulator itself can correctly read this bad tape (so I couldn't take notice, that this tape is bad), but any other emulators (or tape-handling tool) can't read this file.
Yet finally I fixed it: https://retrojatekos.itch.io/speccy-snake-10t-correct-tape-version
Many thanks for the bug-report! :-)