stan_nlmer formula and fixed effects

Summary:

Problem in specifying fixed effects in stan_nlmer formula.

Description:

Following the documentation of stan_nlmer the formula needs to be in the same form as the nlmer one: a three-part “nonlinear mixed model” formula, of the form resp ~ Nonlin(...) ~ fixed + random (from the nlmer help page).
I can't find a way to specify the fixed effect variable. When I do it, after a long list of rejected values I get:

Chain 1: 
Chain 1: Initialization between (-2, 2) failed after 100 attempts. 
Chain 1:  Try specifying initial values, reducing ranges of constrained values, or reparameterizing the model.
[1] "Error in sampler$call_sampler(args_list[[i]]) : Initialization failed."
error occurred during calling the sampler; sampling not done
Error in check_stanfit(stanfit) : 
  Invalid stanfit object produced please report bug

Also if I try to specify my own function with the suggested deriv option (check example 3 of the nlmer help page), I'm not able to fit the model with stan_nlmer.

I suspect is a long standing problem of nlmer..

Reproducible Steps:

Here an example of the problem with the code you have in your help page

data("Orange", package = "datasets")
Orange$circumference <- Orange$circumference / 100
Orange$age <- Orange$age / 100
#define a new to use as fixed effect:
Orange$fixed.eff <- ifelse(Orange$Tree=="3"|Orange$Tree=="5",1,2)
#try to include it following the formula definition:
fit <- stan_nlmer(
  circumference ~ SSlogis(age, Asym, xmid, scal) ~ fixed.eff + (Asym|Tree), 
  data = Orange,  
  #for speed only
  chains = 1, 
  iter = 1000
)

RStanARM Version:

packageVersion("rstanarm")
[1] ‘2.21.2’
packageVersion("lme4")
[1] ‘1.1.23’
packageVersion("rstan")
[1] ‘2.21.2’

R Version:

[1] ‘4.0.2’

Operating System:

OS X 10.15.6

Start with the reproducible Orange example in the issue and the stan_nlmer documentation, then compare its three-part formula handling with the nlmer help example and deriv option. The issue names no source files or tests; done means establishing whether fixed.eff and a user-supplied deriv function are supported and clearly characterizing the observed behavior.