I haven't yet found anything about lacosamide in the treatment of muscle spasms. Like other antiepileptic drugs, it does have some possible side effects but if it's working well for you, why not use it?
I did find a Cochrane review abstract about lacosamide for treating neuropathic pain. And prescribing drugs for so-called "off-label use" seems to be a common enough practice.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8406928/