I had a shitty childhood with as-yet undiagnosed autism and ADHD. Teachers didn't try to understand me, my mom outright called me "weird" and asked "why can't you be normal?" all the time, and classmates picked on me for any reason they could think of. But the phrase "You're very strong," is something I occasionally heard from random adults. From the first time I heard it (around age 11), I was confused at what people were talking about. I would think, "I spend every afternoon crying, how can I be 'strong'?"
When I became a teenager and friends talked about self-harm and suicide, I didn't get it. A school counsellor I talked to called me "strong" again in that context, but to me I thought, "I'm just trying to survive, and that's 'strong'? Not doing a thing I consider stupid and pointless is a strength?" I knew the word was a compliment, and I didn't get many of those so I didn't dig into it. But the way I saw myself was as a weak kid, who must be a terrible person since I was in trouble and picked on all the time without understanding why.
As an adult, I see it now. I remain "myself" despite the pressures around me. Becoming an adult and seeing how many people don't navigate with a strong moral compass, who are swayed by disinformation and hate with ease, who have little idea of who they really are, it makes more sense.
But as a kid? Those traits were liabilities. I didn't conform, and authoritarians (both adults and child bullies, AKA boot-lickers and mini-boot-lickers) have a big problem with that.
It's a strange compliment to get sometimes, but it did stick with me. I just wish those who use the term would explain a bit of what is so strong about the person they're complimenting. Trait and behavior-specific praise is the way to go. I wouldn't be surprised if many kids hear compliments like this and don't know what they really mean.