Advice Needed: Water Pooling and Seeping Under Shingles Where Flat Roof Meets Sloped Roof

I’m dealing with an issue on my roof and could use your advice.

There’s a flat section of my roof (TPO membrane) that covers a patio, and it connects to a sloped, shingled roof. The water from both roofs drains through a few drains, which I try to keep free of leaves. However, during a recent heavy rain, the drains got backed up due to leaf buildup. This caused water to pool on the flat TPO section and then seep up under the shingles where the two roofs meet.

Here’s a breakdown of the situation:

The distance from the skylight to the edge of the shingles is about 20 inches. The TPO membrane overlaps under the shingles by about 12 inches. The slope of the shingled roof seems too shallow, creating surface tension that allows water to creep under the shingles and into the decking below. From left to right, the roof layers are: TPO (flat roof), underlayment, and shingles, with decking beneath it all.

I’m looking for advice on the best fix to prevent water from seeping under the shingles in the future. Aside from keeping the drains clear, here’s what I’ve been considering:

Installing additional flashing where the TPO meets the decking and using sealant to secure it to both the decking and the TPO. Questions:

Is it okay to use sealant on TPO to attach flashing? Are there other solutions or best practices you’d recommend for this kind of issue? I’ve included images for reference:

Image 1: Shows where the issue is occurring (the lowest part of the flat roof where water pools). Images 2 & 3: Profile views of the problem area. Image 4: Cross-section showing the TPO, underlayment, and shingles. Any suggestions or insights would be greatly appreciated!