There are some older cultivars and maybe some new thornless varieties that will revert to spines and prickles with root cuttings. Nothing to do with planting in proximity though…at least to my knowledge.
So its possible with vole damage or root damage to have a sucker that is spined… but its rare… and maybe those cultivars arent in the market any more. I dont think any varieties in the trade nowadays have reversion.
Its more likely at some point to have a seedling from a bird or animal dropping that will be a mutation of its parent…which could result in a spined variety even though the seed was from a spineless cultivar.
I have a seedling from a thornless boysenberry going now that is ultra thorny… i have no idea what kind of berries it will have… i just know that its not growing like a trailing boysenberry and doesnt even have the small spines of a thorned boysen…its a mutation. Could be terrible…could be awesome… thats just how nature works.
If you want to nerd out…this is a good article explaining the process and science of breeding thornlessness and in some cases there may be a chance of reverting.
Most everything that has happened with thornlessness is due to a sport of Loganberry found in 1933 ‘American Thornless Loganberry’… if not for that… hard to know if we would have thornless cane fruits today…alot of breeding has resulted from that seedling.