Yep my DS fell off the centiles, he came out of hospital at 5lb ish and was only 15lb at a year old. He had severe vomiting from Reflux and CMPA, but for some reason all the health bods didn't seem to realise that constantly vomiting would lead to little weight gain.
Once when he was in hospital at 5 months after yet another bout of heamatamesis, they insisted on him being fed in front of them as the hinted they thought I wasn't feeding him , and as they do charmingly pointed out, I wasn't exactly skinny myself-- said the doctor busting out of his shirt
So yes they did try to push me to top up/bottle feed. I did point out that it wouldn't stop him vomiting. While I was in hospital I used up my stock of milk I had frozen for the milk bank.
Now even though they didn't use the actual 'failure to thrive' words, I knew that's what they meant. They eventually backed off when they finally admitted he was a thriving healthy (almost) baby. Every scan (heart, brain, kidney, liver etc) never showed issues. Only his stomach/oesophagus showed signs of damage from all the vomiting
They were always perplexed at his size until they knew his Dad was 9 stone