Late messages after a tx_time tagged message

[Anthony B.]

Hello all,

I am using timed commands to send messages at a specific time, using GNU Radio 3.7.14.
To achieve that, I add a tx_time tag to my message just before the USRP Sink, slightly in the future, and it works fine.
What I'd like to achieve now is to only send specific messages using the tx_time tag, in an asynchronous way.
I just want the other messages to be sent as soon as possible, without timed commands.

What happens currently is the following behaviour:  
- the message n is sent successfully, with a tx_time tag
- the message n+1, without a tx_time tag, is considered late by UHD (printing "LLLLLL") when it is about to be sent

My interpretation is that the USRP Sink keeps the tx_time tag of the message n for all the messages following it.
Is it possible to clean the tags on the USRP bloc between the messsages n and n+1 ?
Or UHD is only expecting an implementation that updates the tx_time tag for the next burst, and so all the messages have to be tagged before being sent ?
Is it possible to mix messages that are timed with the tx_time tag, and in the same flow send others that are not ?

Any suggestion would be appreciated.

Best regards,

Anthony