Could you upload that replay? Watchdog seems to have changed its way of working at least once because last time I tried some random skywars replay this is what I was experiencing.
I can't quite remember what the specific conditions are but iirc spectators should all be completely invisible during replay. Please list all mods with their respective version (including Forge).
The only reason, I can think of, for players not to show up in the Player Overview is that they are technically NPCs (meaning that they have special UUIDs commonly used for NPCs rather than actual players). These are hidden from the Player Overview by default. To show them, hold down Ctrl while opening the Player Overview.
Okay, I've uploaded the replay to my profile, the match I used is near the end, between those markers. I'm not exactly sure how to link to it, but it's on my profile: https://www.replaymod.com/user/CircuitLord
I'm using MC 1.8.9, with Forge 11.15.1.2318-1.8.9
I use autotip, fullscreen windowed, MouseDelayFix, Optifine, Orange Marshel's mods, 5zig, and a resource pack organizer.
Let me know if you wanted anymore info.