I checked, and I do have the 2 meter signal of the SI5351 3rd harmonic. It goes away on CW TX, confirming to me it is the LO Clock2 signal, as that is changed to the actual frequency on CW TX.
As Jay noted, it is quite strong. My ubitx v4 has the relays replaced, and I have verified the harmonic reduction after the replacement. It is in a metal case, however, I do not have a good ground in the shack, just a copper water pipe. If I use my hand as a temporary RF choke around the antenna coax, I can get a significant reduction in the signal strength on the Kenwood TM-D710G, the squelch will drop out. Based on my simple test, ferrites on all lines into and out of the rig would be the answer.
Best guesses at this point.