I'm leaving this for a developer with more knowledge of the particle system. My findings so far:

• There seems to be a relation between the described problem and what is saved on disk when enabling disk caching. When using a scene frame range 1-5, in 2.79 there are files xxx_000000_00.bphys to xxx_000005_00.bphys. In 2.80 that first file with zero index is missing.
• BKE_ptcache_bake(): in 2.79 pid->totpoint(pid->calldata, 0) returns the number of particles; in 2.80 this returns 0.
• BKE_ptcache_write(): same as above, resulting in cfra=0 not being written, which means that the 'info' data in the particle cache isn't written.
• Forcing pid->totpoint(pid->calldata, cfra) to return 1000 (by hard-coding psys->totpart = 1000) fixes the reported problem, as well as causes the zero-index bphys file to be written.

I've managed to get the correct number of particles by applying the following patch.

diff --git a/source/blender/editors/physics/physics_pointcache.c b/source/blender/editors/physics/physics_pointcache.c
index bc2f1d6cef6..d68f076bb2c 100644
--- a/source/blender/editors/physics/physics_pointcache.c
+++ b/source/blender/editors/physics/physics_pointcache.c
@@ -38,6 +38,7 @@
 #include "BKE_pointcache.h"
 
 #include "DEG_depsgraph.h"
+#include "DEG_depsgraph_query.h"
 
 #include "ED_particle.h"
 
@@ -165,7 +166,7 @@ static PTCacheBaker *ptcache_baker_create(bContext *C, wmOperator *op, bool all)
 
   if (!all) {
     PointerRNA ptr = CTX_data_pointer_get_type(C, "point_cache", &RNA_PointCache);
-    Object *ob = ptr.id.data;
+    Object *ob = DEG_get_evaluated_object(baker->depsgraph, ptr.id.data);
     PointCache *cache = ptr.data;
     baker->pid = BKE_ptcache_id_find(ob, baker->scene, cache);
   }

This does fix the missing zero-index bphys file, but still doesn't solve the render issue.