No, they're not. Code sharing has nothing to do with coordinating fares. While UA has come to general terms on a joint venture with Copa and Avianca, I don't see any evidence that they've actually applied for antitrust immunity yet, making the coordination of fares illegal. In fact, if the fares were coordinated, the behavior you're seeing would be less likely --currently, that can happen due to either fare or inventory differences, but if the fares were the same across carriers, then it could only happen due to inventory differences.

The codeshare agreement is still in place, but it looks like UA has dropped most or all of its Copa codeshares for the next several months. They seem to start around Memorial Day (late May).

Incidentally, whereas you can route UA economy fares via a CM flight number, it appears that business class fares require a UA code; otherwise, you get a fare break in PTY. (At least, that's what I'm seeing on the search that I did -- May 25 to June 14, IAH to MDE). That's why you're seeing the higher price -- it looks like UA has negotiated the ability to use CM flight numbers for economy class travel but not business class travel.

Whether this was intentional or an oversight, who can say? (Incidentally, even if they had a joint venture, if they wrote their fares this way, you'd still get hit with the price increase).