This may be a (possibly unavoidable) problem with that particular type of GPS app on that particular device model.
An app using lots of graphics GPU features at high display frame rates in addition to continuous access to the GPS radio plus with the cellular radio power turned up in weak cell signal areas might consume more current than the battery charge circuit is capable of replenishing on some device. e.g. even with a 10+ watt USB charger, the iPhone (operation plus charging) circuitry may only be able to use 5 watts total to avoid overheating.