I believe this has been fixed in 4.2
Issue was that the per contact frequency rule for text is defaulted to blank. Mautic reads that as only allowing 1 text per day and ignores the frequency settings in the twilio plugin. Upping the frequency allowed in the per contact preference centre fixed the issue. You can globally fix the frequency issue by either upgrading to 4.2 or applying the PR fix found here Mautic 4 - Twilio, can't send more than 1 text message, it breaks after the first sms · Issue #10637 · mautic/mautic · GitHub
Big thanks to @mikew and @joeyk for taking time out to help find the issue and the fix