Vim9: cannot write block in autoload script sourced while calling omni completion function

[lacygoill]

Describe the bug

In Vim9 script, we cannot write a block in an autoload script sourced while calling an omni completion function.

To Reproduce

Run this shell command:

vim -Nu NONE -S <(cat <<'EOF'
    vim9script
    set rtp=/tmp/some
    var dir = '/tmp/some/autoload'
    mkdir(dir, 'p')
    var lines =<< trim END
        vim9script
        def omni#func(findstart: bool, base: string): any
            if findstart
                return 1
            else
                return ['match']
            endif
        enddef
        {
            eval 1 + 2
        }
    END
    writefile(lines, dir .. '/omni.vim')
    &l:omnifunc = 'omni#func'
    startinsert
    feedkeys("\<C-X>\<C-O>")
EOF
)

The error E565 is given:

E565: Not allowed to change text or change window: {

Expected behavior

No error is given. match is inserted in the buffer.

Environment

• Vim version: 8.2 Included patches: 1-3215
• OS: Ubuntu 20.04.2 LTS
• Terminal: XTerm(353)

Additional context

No issue if the block is removed:

vim9script
set rtp=/tmp/some
var dir = '/tmp/some/autoload'
mkdir(dir, 'p')
var lines =<< trim END
    vim9script
    def omni#func(findstart: bool, base: string): any
        if findstart
            return 1
        else
            return ['match']
        endif
    enddef
END
writefile(lines, dir .. '/omni.vim')
&l:omnifunc = 'omni#func'
startinsert
feedkeys("\<C-X>\<C-O>")

no error

---

No issue if the omnifunc is moved in a non-autoloaded script:

vim9script
def g:OmniFunc(findstart: bool, base: string): any
    if findstart
        return 1
    else
        return ['match']
    endif
enddef
{
    eval 1 + 2
}
&l:omnifunc = 'g:OmniFunc'
startinsert
feedkeys("\<C-X>\<C-O>")

no error