It works as expected when the both has threaded mode disabled. However, once I enable "Threaded mode" and "Disallow users to create new threads", the following happens:
Telegram Android continues displaying the keyboard that was last sent while the bot was in non-threaded mode. New keyboards are ignored. If a chat never had a keyboard from the bot, it doesn't appear.
Telegram Web A displays the keyboard, but marks it as inactive, disallowing sending a response. Telegram Web K displays the keyboard, but doesn't allow to send a text response to the non-topic thread.
The keyboard works on Telegram Desktop 6.5.1 x64 as expected.
Steps to reproduce
Send a message with an reply keyboard from a bot with threaded mode enabled:
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