Here the same problem, but I believe I have found the reason for this issue. I have this problem since the latest update, and have lost more then one one day to find the reason for the problem.
I'm using tvmosaic plus with my raspberrypi. Tuners are two in use, TV-Butler and Hauppauge WinTVHD Dual (only one tuner with Kernel 4.14).
It seems the TV-Butler is not working with the latest software, but tv-mosaic is always using the TV-Butler as standard tuner. If you remove the channel list from the TV-Butler, than the second tuner works without problems.
Could you please repeat this to confirm my workaround?
!UPDATE! I have removed the TVButler stick from the USB-HUB and the reconnected to an other port. After this I have copied the channel list from the Hauppauge tuner (with same search preferences), and now it seems it works.
General discussions about TV Mosaic application.
Surprise! That simple trick was the solution. I've just changed the USB-Port for the TVButler on my Synology and restart. Thanks for your help!