I wonder if yours is a different cultivar. Mine stays in a clump, and does not spread by underground rhizomes the way the mints do. However, I've found that the many seedlings that have gone rogue in my uncultivated areas have over time become less fragrant than their ancestors were! I actually bought some lemon balm last weekend to replace the rogues. Some of those rogues are being planted along with peppermint and oregano at the base of my chicken run.
https://www.gardeningknowhow.com/edible/herbs/lemon-balm/growing-lemon-balm.htm
If not carefully controlled, lemon balm can quickly become invasive in the garden. Often, people mistakenly think that lemon balm is invasive due to its roots, like its cousins peppermint and spearmint, but in fact it is the seeds of the lemon balm plant that cause this herb to suddenly take over a garden. Removing the flowers of the plant as soon as they appear will make your lemon balm far less invasive.
https://www.gardeningknowhow.com/edible/herbs/lemon-balm/growing-lemon-balm.htm
If not carefully controlled, lemon balm can quickly become invasive in the garden. Often, people mistakenly think that lemon balm is invasive due to its roots, like its cousins peppermint and spearmint, but in fact it is the seeds of the lemon balm plant that cause this herb to suddenly take over a garden. Removing the flowers of the plant as soon as they appear will make your lemon balm far less invasive.