please upload a screenshot of your Progress / today practice stack.
I always see differences between the left hub and orange practice words vs progress / today statistics (it takes time that the left hub panel re-syncronizes).
No, Lingvist can't show you "old" cards for those learned new cards, IF those words to not meet their SR due and "re-learning (error repetition)" requirements.
So if your repeat / practice stack is almost 0 (no one knows why sometimes the system throws out new cards when the count is 10-15 or more), you have to wait a bit.
On Memrise the intervals are 4-12 hours.
When I came back from a 45-90 minute break at least 60 words had been refilled...
If you answer questions for NEW words wrong, there will be another (annoying 3-5 or probably even 7 times, I was not counting) "repeat" algorithm for those questions answered incorrectly.
These intervals seem to be tuned for short-repetitions to make my brain explode.
Unfortunately all repetitions, either in minutes and hours and also the error repetition counts are very intransparent compared to other software.
However, I think there might be another bug in the current "Lingvist free" system starting 2018 that once you hit the 50 words limitation, that old (due) words can not be reviewed, as announcing in their newsletters and pricing matrix.
It will take some time to fix the known bugs for their development time.
We just need to be patient....
I am sure they are working already on it.
A screenshot might help them to understand and replicate, what is going on on your site.