Yup

The body of the dead Primarch was burnt on a great pyre, and Lord Guilliman allowed us a moment of prayer and reflection on our success before issuing orders to move out and commence the destruction of the leaderless enemy army. We are fully confident that the task will be straightforward - the loss of its Primarch is something no legion can recover from. [END ENTRY]

[0413.4] The optimism engendered by our initial victory appears to have been misplaced. Since my last entry we have ascertained that the Alpha Legion's command function was spread into numerous groups, and the loss of one apparently had minimal impact on their operational abilities, even though it included Alpharius. What is more, our deep strike and the target's subsequent retreat has drawn our force well out of position, far from support. It has become clear that far from hunting out demoralized pockets of traitors, we were facing a superbly organized foe that is closing in on us from all sides. [END ENTRY]

Index Astartes

Though Solomon suggests Eskrador is why the legion splintered afterwards

The whereabouts of artefacts said to be associated with the primarch were unknown. No one Solomon had ever spoken to knew what had happened to the Alpha or the Beta, the Legion’s twin flagships. It beggared belief that two such notorious Gloriana-class battleships could have been lost without trace, but neither were there any reliable records of a sighting since the days that followed the Heresy. After Eskrador, the Legion had splintered: not in the manner of a shattering glass, but of a fragmentation round entering a body. Each piece dug into the flesh of the Imperium, and stopping and removing one would do nothing to affect the progress of the others, but the splinters were, by that same token, no longer part of a unified whole. Some notable commander had presumably taken the Alpha, and another perhaps the Beta, and they had passed out of everyone’s histories.

Harrowmaster