Welcome in the Stan forum!
Did you have a look at the Stan code to check what link function brms uses for the lognormal?
(Brms generated Stan code for complex models can be hard to parse. If this is the case, it can help to start with a simpler model).
If it is the identity link function, as this discussion here Understanding shifted lognormal parameters and priors suggests, that could be part of the problem.
Also:
If I remember correctly, brms uses flat priors for regression coefficients other than the intercept if not specified otherwise. To get a better behaved model you could put priors on the other regression coefficients.
It’s also not necessary to use ‘0 + Intercept’. You can just use ‘1 +’ … and specify a prior for the intercept.