It very well could be the issue, although I can't imagine why it would be. If possible, disconnect both sides and see if the issue goes away.
As far as flashers go, the EP27 I believe is your only option. That being said, if there is an alternative, I've never seen one nor have I heard one mentioned; although that doesn't mean it 100% for sure non-existent.
As far as loads go, a standard 5mm LED such as those draw about 20-30 milliamps each at 2 volts, usually. Let's assume 20 milliamps and 2 volts exactly for sake of example. Even if they were all wired together in parallel, that's a whopping 180 milliamps per side, or .18 amps which is incredibly puny.
In comparison, a similar lamp from SBL (compared to the larger 31 series bulbs pictured), it only draws 320 miliamps on it's brightest mode of two. 50 milliamps on the low side.
On one side, the maximum load would be 820 milliamps, continuing to assume that all LEDs on the one mirror are wired in parallel for example's sake. That's less than one amp.
Again on SBL the maxium load is 15 amps, however this is not the EP27 you used.
So, bringing up the Novita brand EP27 off autozone, it lists '2 turn, 6 hazard' with 162 watts maximum. 162 watts at 12.8 volts is 12.65 amps of current.
So taking that spiel I just threw at you, I highly doubt that the EP27 is being overloaded. However, something to look at is the 2 turn, six hazard rating. They aren't really clear about what this means, but I'm guessing it means a maximum of two turning lamps and a maximum of six hazard lamps. Taking the hypothetical 180 milliamp rating from before, it could be seen as another lamp thus increasing the load on it, but I still doubt your mirrors are the problem.
do you have a howto for that LED strip there? I'd like to do something similar when I get 06 mirrors.
Two things worth trying first. Disconnect your mirrors, both sides, and try again. See if that helps the issue. If it does in fact make an impact then I would inspect the board for a possible solder bridge.
The other thing to try is to take your Novita EP27 flasher back and either exchange it for another one or buy a different brand of EP27.
This is the EP27 I use and my 99 has several turn signals in LED. https://www.superbrightleds.com/more...Specifications
Hopefully something in there gets you some results.