'tmsh config sys load' causes monitors (healthchecks) to fail

Not directly answering your question, I would like to provide some information instead. The default send-string uses the HTTP v0.9 syntax and that's why there is no version information specified.

I can imagine the situation in which a system uses the default value when no customisation is specified.

And an HTTP monitor without a send string is something new and interesting to me, for I had thought I could not rely on something like this myself.

Thanks for sharing the info that this has been confirmed as a bug and I sympathise with your unexpected plight, which I would never wish myself into.