Came across this thread accidentally and it seems that I’ve been having the same issue. You can just read my post here: http://forum.mikrotik.com/t/chateau-lte-12-losing-ca-band/141699/7
TL;DR - Mikrotik Chateau LTE18 tries to use 4x4 MIMO even if you have enabled external antennas. And if your signal for internal antennas 3 and 4 is barelly registering then tower will drop those bands.
I did add another pigtail for the Mikrotik LTE18 modem (replacing the internal antenna) and attached another external antenna. In total I had 3 yagi type antennas and it seems that the rate of CA bands disappearing reduced by atleast 70%.
Now I got my hands on Mikrotik ATL LTE18 and it runs very stable with CA bands, because all the antennas are exposed compared to LMT provided mikrotik LTE18 which only exposes 2 external antennas by default unless you disasseble the unit and add your own pigtails. It still managed to loose B1 band at some point during the day, but it seems that it did not affect my speed…
I also had some communication with LMT support and eventually got a response from a tech guy which stated that if you have low SINR/MCS then the band will be dropped from the tower side. He did not state the threshold, tho.