I believe its better to turn off collisions on the mesh, and instead create a invisible part behind inside the mesh with the shape of the part being close to the arch, and have that be the collision box.
This is a hit-box issue.
To summarise:
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This is caused by Roblox making a less detailed version of the mesh. This can be visualised using this plugin.
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There is no way to fix this without using different software.
Recommended Solution:
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Make the problematic mesh CanCollide False
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Place down multiple parts and resize them to match the intended collision walls
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Set the part āTransparencyā to 1
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I recommend you group them all together for easier editing in future
Memory Consuming Solution:
- Set the part to PreciseConvexDecomposition. This is a relatively new tool which is very memory consuming so should be used in moderation. Read the DevForum post here.
Yep!
Thatās what I said! 
I understand, but you recommended unions which are as you said memory heavy, and its better to use parts in my opinion, even if it doesnāt match the shape exactly, its better to have no unions at all.
Iām pretty sure I said āNon-Unionedā
Oh my bad, I misread your post, you are correct then.
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roblox has alot problems with hitboxeās i think. i had alot problems with the same thing.