I see this behavior using a ROKU Ultra (W10 server Hauppauge quad PCI tuners, wired Gb connection) all the time. Time shifting has always been sketchy and there’s another thread here with more examples - Start Watching Recording in Progress from Beginning(Failing) - #972 by rakstr_sbcglobal_net
You may also notice that if you’ve been watching a station for an extended period of time and attempt a time shift it will go back previous shows and/or random spots. 
I used to think it was only while you were recording but it doesn’t matter. The behavior is the same whether the show is set to record or not…
