I think it'll be fine, like others said it's a nice clean break.
The question I have is do you have any sand sifters? Goby/pistol shrimp pair, nessarius snails?
If so that's most likely what caused your rock to fall if you put your sand bed in first followed by your rock, that is if the whole structure fell. They dig burrows under the surface then one day BOOM!! as you experienced avalanche. If it was a midway fall then you have balance issues.
I would rescape your tank as much of a pain as that is to be able to take your time to balance pieces so it doesn't happen so easy and you don't have to worry about what Kathy said about a piece cracking your glass, that could be the biggest nightmare you'll ever have to go through and while you're at it plant your bottom layer of rock directly on the tank bottom by moving the sand as little as possible but enough to plant the base of the rocks on bottom.
I never liked the idea of epoxying or puttying together my rock, it makes it all one big piece in a way and even if it doesnt it will add ridges that make rescaping in the future with that same rock a bit limited on how you can rest what rocks how on each other. You can pull out the rock, drill the pieces and then use either driveway markers or clear acrylic rod to join the rocks together but have them be removable just way more stable.
Best wishes and I'm glad it worked out like it did with the new frag.
