When reviewing code, I discovered that Master_info::heartbeat_period is a float.
Yes, the 0.1 * 3 != 0.3 kind of float.

Modding MTR test rpl.rpl_heartbeat_basic, I can confirm that it indeed only supports 7 decimal digits:

• 4294966.999 shows up as 4294967.000 in both SHOW STATUS and SHOW REPLICA STATUS.
• 4294967.001 does not trigger the “4294967 exceeded” error (whereas “4294967.999” does).

The test (with the expected .result, not actual): https://github.com/MariaDB/server/compare/10.5...mdev-35879.test

This variable should be a double for enough precision or, better, a DECIMAL(10, 3) (or internally stored as a uint32_t of milliseconds).