Showing strange behavior with zig StringHashMap

testProto.zig

const std = @import("std");
const testing = std.testing;
const ArrayList = std.ArrayList;
const StringHashMap = std.StringHashMap;

pub const Protobuf = struct {
  buf: ArrayList(u8),
  pos: usize,
  len: usize,
  allocator: std.mem.Allocator,
  const Self = @This();

  pub fn init (input: []const u8, allocator: std.mem.Allocator) !Protobuf {
    var buf = ArrayList(u8).init(allocator);
    try buf.appendSlice(input);
    return .{
      .buf = buf,
      .pos = 0,
      .len = buf.items.len,
      .allocator = allocator,
    };
  }

  pub fn getValue(self: *Self) u8 {
    defer { self.pos += 1; }
    return self.buf.items[self.pos];
  }
};

pub const A = struct {
  pbf: Protobuf,
  data: StringHashMap(B),
  arena: std.heap.ArenaAllocator,
  const Self = @This();

  pub fn init (buffer: []const u8, child_allocator: std.mem.Allocator) !A {
    var arena = std.heap.ArenaAllocator.init(child_allocator);
    var allocator = arena.allocator();
    var pbf = try Protobuf.init(buffer, allocator);

    var a = A{
      .pbf = pbf,
      .data = StringHashMap(B).init(allocator),
      .arena = arena,
    };

    try a.data.put("testA", B.init(&pbf));
    try a.data.put("testB", B.init(&pbf));

    return a;
  }

  pub fn deinit (self: *Self) void {
    self.arena.deinit();
  }
};

pub const B = struct {
  pbf: *Protobuf,
  data: ArrayList(C),
  const Self = @This();

  pub fn init (pbf: *Protobuf) B {
    return .{
      .pbf = pbf,
      .data = ArrayList(C).init(pbf.allocator),
    };
  }

  pub fn getC (self: *Self) !C {
    std.debug.print("\npbf contents: {any}", .{self.pbf.buf.items});
    return try C.init(self.pbf);
  }
};

pub const C = struct {
  x: i32,
  y: i32,
  const Self = @This();

  pub fn init (pbf: *Protobuf) !C {
    return .{
      .x = pbf.getValue(),
      .y = pbf.getValue(),
    };
  }
};

test "run" {
  const exampleData = [_]u8{ 0, 1, 2, 3 };

  var a = try A.init(&exampleData, testing.allocator);
  defer a.deinit();
  var b1 = a.data.get("testA").?;
  var c1 = try b1.getC();

  try testing.expectEqual(c1, C{.x = 0, .y = 1 });

  var b2 = a.data.get("testB").?;
  var c2 = try b2.getC();

  try testing.expectEqual(c2, C{.x = 2, .y = 3 });
}