I believe that you have answered your own question. You are a tester and not a developer, also you have the title of a senior developer. That's pretty much it, administration "error". But:
In my community there is saying (roughly translated): you are a developer if you are getting things done. Eather that be by some copy/paste of some code, asking a friend to do it and then you submit it... whatever it takes, just make it work.
Reinventing things are not time/cost- efficiant. If there is a solution on the net, we use it. Doing those algorithms coding challenges are now done at home and mostly for fun.
On the note of the bugs. Trade-off. Writing your library where you know how things work is slow, but you can debug them. Using other code is quick but.. you get the point. Luckily, we don't work alone and any bug is fixed by us, or we ask someone to fix that for us.