The basic literal meaning of to encounter is to meet up with unexpectedly. That unexpectedly coming to face someone (or something) occurs the very instant it happens, and so in that sense "to encounter" doesn't really go well with the continuous.
This is certainly unidiomatic in American English:
On the way here, I was encountering a detour.
Now, let's say you want to describe a situation where multiple drivers over a period of several hours encounter a fallen tree. In that scenario you could use the continuous because you're referring to what is happening over a span of time, not in an instant:
Motorists on Route 9 are encountering a fallen tree.
And you could use the continuous if you wanted to stress the fact that you're trying multiple different approaches but they all hit the same obstacle:
I am trying to remove a stain from the upholstery but am encountering the same problem: every cleaning product I find contains at least one known carcinogen.
But absent such a context the continuous can sound "off" with "encounter".