Here are the Roblox experiences I found that connect to Sonic vs Xain from FNF:

Experience / video| What it shows
---|---
FNF Multiverse - Third Party (Vs. Sonic.exe Rerun) FC| ROBLOX 3
Final Escape Sonic Vs Roblox (Sonic.Exe) - FNF 5| Sonic vs Roblox crossover content, useful if you meant Roblox FNF Sonic-style experiences.
FNF Dark Fleetway Sonic vs Xain Breaking Point Playable! HARD 1| Shows the Sonic vs Xain mod gameplay, but it is not a Roblox experience page.

What this means

The search results point more toward Roblox videos featuring FNF/Sonic content than a clearly named Roblox experience specifically titled “Sonic vs Xain.” The strongest direct match in the results is the FNF Sonic vs Xain gameplay video, while the Roblox-specific results are more general Sonic.exe or FNF-related content.

Best match

If you want the closest Roblox-side result from what was found, FNF Multiverse - Third Party is the clearest Roblox experience-style result, but it is not explicitly Sonic vs Xain. If you want the actual Sonic vs Xain mod, the direct match is the FNF video tied to Breaking Point and Lost My Mind.

TL;DR: I found Roblox FNF/Sonic.exe-related content , but not a clearly verified Roblox experience dedicated to Sonic vs Xain itself.