For me, it's usually because I'm tired and my brain is crapping out, or my computer is overheated or the fps is dropping. dodges tomatoes
I don't think I've ever experienced loss 'streaks' out of nowhere, actually, I'm straight up 50/50 but the very few times I experienced a loss 'streak' it was because of tiredness. what's happening in your support games? not that i'm any good at it, i'm an old returning player and a damn casual playing norms and filling all the time these days.
The old way of support was that if you are playing "just good enough" then you are straight up dependent on your team, you do fine if your team is fine, if your team is not fine you don't do fine. If you are playing really good, you should be able to at least give most bad adcs a chance, and even if your adc craps out, be able to carry the team fight with support items that help the team as a whole/give the team enough of an edge to win the team fight and lead the objectives and stuff (ooooh, old aegis, i loved you so) or depending on your type of support, initiate or stun-target the bad guy and stuff. Buuut maybe you're doing that and Idk how it is now though
With the changes I don't know how it is now, though. Hopefully some experienced in the current gameplay main supp player answers.
On the other hand it's entirely possible you won so hard your hidden eloscorethingwhatever changed and went up and now you're matched with players more senpai so it'll need to settle down a little to find the right spot where you're at 50/50