voom mean-variance plot has set of genes where variance increases with expression level

I’m using limma + voom to model an expression dataset, but I’m observing a weird subset of genes where the standard deviation increases with the expression level rather than decreasing as is the case for most genes.

Any ideas for why this is occurring and/or what to do about it? I have found lots of advice regarding oddities at low-expression levels, but not this pattern. Thanks!