I will try and explain my situation as clearly as possible, but let me know if you need any follow up info. Any help would be greatly appreciated!

1. Bitcoin OG here, got Gox'd and Cryptsy'd and (mostly) took a break from crypto for a few years. I still have an old wallet from early 2014 with a few BTC on it. The wallet.dat is a backup of the original wallet I probably don't have anymore.

2. I have the latest version of Bitcoin Core (0.20.1), upgraded from a 2017 version (0.15.1 or something like that) recently.

3. As of a couple of weeks ago, the wallet showed a balance of 4BTC. Blockchain shows a balance of 4.09BTC (!)

4. There were transactions of 0.1BTC, 3.5BTC and .4BTC received by the wallet in 2014 a couple of weeks before the wallet.dat appears to have been created.

5. According to blockchain.com, there was an additional transaction of 0.09BTC that appears to have been sent to the same address on the wallet about 30 minutes before the wallet.dat backup was created. I'm guessing this was me emptying an account before putting the wallet in cold storage onto a memory stick. This transaction did NOT show up in the wallet even though it appeared to have fully synced before I used the wallet again with Bitcoin Core a couple of weeks ago.

I probably made a mistake here by doing this but... 
7. I sent a small transaction of 0.01BTC to Coinbase just to see if it was working a week ago. 

8. The 0.01BTC was received by Coinbase (great!), and showed on the wallet that 0.01BTC was sent.

9. HOWEVER, the blockchain shows that an addition 0.09 was sent to a random address (that looks like a new address - it starts with a "b" whereas all the older ones start with a "1"). The wallet balance and the blockchain balance are now very similar (just 0.001 off) but still not exactly the same.

10. I looked in the logs in Bitcoin Core and indeed there is a line that says "CTxOut(nValue=0.09000000" above the other line for where the 0.01BTC was sent (minus the fee) i.e. "CTxOut(nValue=0.00997324". BUT I ONLY TOLD THE WALLET TO SENT 0.01BTC (minus the fee), and the wallet only shows 0.01BTC being sent. Very bizarre!

11. Blockchain now shows that from the first 0.1BTC transaction made, all of it is spend (I've only sent 0.01BTC, but according to the blockchain, the other 0.09BTC was also "spent" from that original 0.1BTC transaction).

12. The 0.09BTC in the new address that I did not send does not appear to have been "spent". Do I have access to that somehow???

QUESTIONS:

What's going on?!!! Where did the 0.09BTC go, and can I get it back?

Why didn't the 0.09BTC transaction sync up when I finished downloading the blockchain? (Before I finished downloading the blockchain I hypothesized that it just wasn't fully confirmed before the backup wallet I'm now using was created which would explain why it wasn't on the wallet originally)

Is there anything fishy going on? I would have had the old 0.15 version of Bitcoin core running for a bit before realizing I needed to upgrade to 0.20.1. Could this have caused an issue?

Can I still use this wallet? I'm nervous that if I make another transaction it will send coins to a random address again!!

Thanks in advance for any insight!