Hello Albert:
Sorry for the time with no responses.
Are you still having this issue?
Typically the BREAK character is formed by logic 0s, ¿so are you using the TX inverted mode? In that case you would need to just leave the line idle for the required 8.33 ms to get the 0 logic mark you are expecting, or force an idle character by clearing and setting C2[TE] as explained in the manual:

I hope this helps.
Best regards,
Jorge Gonzalez
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