I cannot reproduce this with a variety of tests. Please describe your MC environment in great detail. Where are the files? Where are they being played from? Where are you playing to? DLNA? Local? All that stuff.
In my case music files are stored on and played from a NAS drive (got an issue here also since MC26.0.18, more on that later) in a DLNA set-up, using both a laptop and JRemote as "control points" - depending on what suits the situation the best and whether I'm working on or otherwise use my laptop. The issue persists regardless of whether I play FLAC of WAV files (which are the file formats my music is stored in). My core server is an older HTPC set in server mode in MC26 (not Media Center), like I use to with previous MC versions as well. OS is Win10 64-bit.
The problems began in the process of upgrading to MC26 (from MC25), where during the end of installation I was given a message via a pop-up that there was a problem retrieving library files ("Critical error" etc., can't remember the exact wording), and I couldn't go on from there. I then opted to "start from scratch" (the last of 3 options given to me) to get the upgrade finalized, after which MC spent some time importing music files anew. So far so good.
However, when I was about to prepare playback and get the music flowing from the speakers, I wasn't able to choose my usual NAS-drive in 'Playing from' (in my case named "Malick") in that no files were attached to it ? I then chose 'Main Library' and all the files were localized here (they're from the same source, apparently, the "Malick" NAS-drive), and music now played fine - except, like the OP of this thread, I had the problems mentioned before.
I just tried downloading earlier MC26 builds, but for some reason was denied access in trying to do so..
EDIT: On my laptop from where can control playback, other than JRemote, the player display is correct in stating the artist playing, title and all that (contrary to MC on the core server, where it says "0:00 / Live"), but when I try and seek into the track, placing the cursor over and clicking on the time-bar, playback is unaffected and just keeps on playing from where it initially started. Moreover, only a single track can be played; playback won't continue into the next track.
Via JRemote the seek function into a track is disabled as well, but here playback automatically continues into the next track.