Problems with low ESS when using brms::brm_multiple

Hi,
I am experiencing very low ESS when using brm_multiple (some values in the region of 100 when I would expect 2000). Here are details of relevant information, I would be really grateful for some advice. Thank you.

  • Windows 10, R version 4.1.3, BRMS version 2.16.3, MICE version 3.14.0

  • Data with 2700 observations has approximately 14% missing data, all of class numerical. Used mice to create 30 imputed datasets using method=“cart”. Plots used to assess quality of imputations and I was satisfied.

  • Data used in a brm_multiple model with one random effect and 40 fixed effects, horseshoe priors applied to 30 of the fixed effects, default priors applied otherwise. Used 2000 iterations (1000 warmup), 4 chains. Specified a high target acceptance criterion of 0.98.

  • If I select one of the imputed datasets and run as a brm model, the ESS and rhats are all satisfactory. However, if I run the brm_multiple model i.e. to pool all 30 imputed datasets, I am left with very low ESS as described above (NB also convergence warnings and rhats>1.05 but as described in the brm_multiple documentation this is likely false warnings due to the chains across different datasets maybe not overlapping)

I am grateful for any advice, thank you.