That’s pretty much how slots are designed to feel. The RTP (say 95–97%) doesn’t mean you’ll get that percentage back every session—it’s calculated over millions of spins. In the short run you can be way up or way down, but the longer you play, the closer you drift toward the house edge.
Casinos don’t need to secretly rig slots, the math already guarantees they win in the long term. The ‘hot start then slow drain’ pattern you noticed is super common because our brains remember the exciting wins and feel the losses more painfully. The only real way to walk away ahead is literally to… walk away while you’re ahead. 😅
However, we will still difficult to win in slots. You rely on your luck to win, no matter how high the RTP. Playing gambling for the short term or the long term will not guarantee you to win but we know that you can lose anytime. Casinos will win from the losing gamblers so they don't have to cheat or change the algorithm of the slots game. That is because many gamblers will be tempted to win but they will lose much money. Casinos are only waiting for their time to collect all of the lose of the money to be their profit. That is why we must be wise when using the money to play gambling.