When combining Pledges, the fastest user waits for the seconde one to move, right after it. Then the combined effect of both Pledges merges into ONE AND ONLY move. The user's move has no effect and only the combined Pledge move has an effect. However, by using Grass Peldge's max variant, the original effect of the move is negated, yet it technically remains Grass Pledge (same interaction as Copycat on a Max move). Thus, the combined Pledge mechanic happens, negates Fire Pledge's damage since Venusaur's Grass Pledge should have the combined effect, and then negates that effect by making it Max Overgrowth.
Check out Fire Pledge dex entry or Bulbapedia page if i didn't make it clear enough.
I just tested it out on the Switch. Fire Pledge worked normally and attacked without waiting for the Grass Pledge-based Max Overgrowth. So it seems it actually is a Showdown bug. Please, admins! Thank you in advance!