I was using a Lebara SIM in my Librem 5 for overseas travel. Then I wanted to try my USA sim again. I switched to USA and back, and now the Lebara sim only has Data, but calling and texts are basically dead. They stayed dead for several days and on both dual boots (crimson, byzantium).
Is this more likely a SIM issue or a cell provider shadowban issue? Seems difficult to debug. USA SIM is still not working overseas here.
Yes. Many moving parts.
VoLTE issue?
“Overseas” is quite large. Remember, L5 modems are not global and not all bands are available. One thing to consider is, that if you are on the move somewhere in that distant country, you may end up to a coverage area there, where you may have not have your modem’s bands available (voice and sms may use different band than data). Easy to eliminate that as a reason though, if you are in the same area where your sim worked previously.
Don’t think it’s a shadowban issue… Do you have roaming on either SIM? I wonder if swapoing with roaming would trigger some backend system to prevent certain aspects being using in the case of a SIM swap to a modem that is already used by another service/carrier (which it should’t do). Being said it shouldn’t do it is also funny because sometimes tech just randomly does weird things, considering the silicon lottery, charged particles, and potential use of roaming, anything is possible…
But SEE events are fairly rare, but being in a plane can increase the avaialvility of charged particles resulting in a higher likelihood of Single Event Errors.
The OP has previously stated that he is in Europe, albeit that “Europe” is still quite large, just not as large.
Switch modem hardware switch to “ON” 
Some thoughts:
1_ BM818 modems are unfortunately regional, and frequency bands likely don’t align well with carriers outside the BM818’s designated “home” region (i.e. North America vs. UK vs. EU).
2_ Some area networks on which you have previously been able to roam in your travels may have had residual 2G or 3G networks still available that happened to coincide with one or more frequency bands of the particular BM818 you’re using.
3_ Carriers (or coverage areas) that require VoLTE to complete calls due to the absence of 2G or 3G network coverage may not recognize a BM818 as certified for VoLTE. In this case you could be right about being disabled by the networks you attempt to roam on as you move about, especially if you’re using a BM818-A1 in the BM818-E1 zone.
4_ If your USA SIM is from an MVNO and not T-mobile or AT&T themselves, international roaming may not even be offered; the MVNOs that do offer it sometimes require that you first deliberately enable it in your account. Ting requires it, as an example. (I think Purism does not offer international roaming with AweSIM/SIMple, unless I’m mistaken.)
As others have commented, without more details from you, there are several possible explanations for your “disabled” Lebara SIM.