BEGIN OF STEPS TO REPRODUCE
Type this command:

printf '%s\n' {0..10000} | : | less

You will probably see that "less" will launch. If it will, then press "q" to quit "less". Then you will see:

[1]+  Stopped                 printf '%s\n' {0..10000} | : | less

I absolutely don't like this message. It is annoying. It requires me to run "fg"

I reproduced this is two debian versions. First is (old) debian stretch amd64 with "GNU bash, version 4.4.12(1)-release (x86_64-pc-linux-gnu)" and less 481. The other is fresh debian sid amd64 (as on May 2023) with "GNU bash, version 5.2.15(1)-release (x86_64-pc-linux-gnu)" and less 590. Details of reproduction in this two environments slightly differ.

Also the bigger number is in {0..10000}, the more likely is to reproduce the bug. In particular, in stretch {0..10000} always reproduce the bug, but {0..10} - does not
END OF STEPS TO REPRODUCE

DISCUSSION
I don't know whether this is bug in bash or in less. Also, let me tell how I actually found this bug. I tried to type something like this:

history | sed 's/^....//' | less

Keep in mind that I have big .bash_history (241119 lines). But "sed" command gave error. I will not get into details, let's assume that I simply mistyped "sed", i. e.:

history | sedd 's/^....//' | less

And I got that annoying "[1]+  Stopped" message. I actually often get into such situations, so this is legitimate bug