Appending two mkv files results in corrupted playback at the second file

Trying to combine two mkv files, a smaller one first, then a larger one concat'd to it. In the resultant mkv, the first file's contents play back as expected. However, the second file's contents result in halted playback. Seeking breaks. The audio stream continues to play. These are both HEVC H265 container encoded mkv's. They both play normally. I tried two video players: VLC and MPC-HC. The behavior is the same.