Birdie is crashing at start

[Fredouille]

Hello,
I have problems to start Birdie Tape using Caprice Forever with "CPC 464 UK" default profile.

When hitting CTRL+ENTER manually, it works most of the time.When using "Auto Play", it crashes all the time. But, "Auto Play" is just an automatic Tape Play + "CTRL+ENTER" keystrikes !!

I have set a breakpoint on &8000.After loading blocks of "BIRDIE.BIN", emulator stops on &8000.
Birdie loader first instruction, at &8000, set stack pointer on &0898.At &8035, loader copies part of code from &8083 to 0, replacing interrupts behaviour.
I have set another breakpoing to &38.

If IT occurs when copy have been done, all seems OK.But when IT occurs before copy is done, then, the default jump is performed to &B939, then LowerROM is activated in &B946.At this point, at first use of SP, program will crash because stack is no longer readable.
So, I do not really understand because it should happen using real CPC !!And I have never seen something related to this problem.

I am not a Z80 expert, Please, is there something wrong in my analysis ?